Investigation and Enhancement of Stability in Grid-Connected Converter-Based Distributed Generation Units With Dynamic Loads

Medium-voltage distributed generation (DG) units can be subjected to a high penetration level of dynamic loads, such as induction motor (IM) loads. The highly nonlinear IM dynamics that couple active power, reactive power, voltage, and supply frequency dynamics can affect the stability of MV grid-connected converter (GCC)-based DG units. However, detailed dynamic analysis and, more importantly, stabilization approaches of GCC-based DG units with IM loads when subjected to the grid faults, are not reported in the current literature. In addition, the literature lacks a thorough study on the effect of the grid strength on the low-voltage ride-through (LVRT) performance of such practical systems. To fill in this gap, this paper presents comprehensive integrated modeling, stability analysis, and LVRT performance improvement methods for GCC-based DG units in the presence of an IM load considering different grid strengths. A detailed multi-stage small-signal model of the complete system is obtained, and the eigenvalue analysis is conducted considering both static and dynamic load modeling. Furthermore, a sensitivity analysis is performed to investigate the effect of the length of the power line between the DG unit and the IM on the stability and LVRT performance of the entire system. Finally, the LVRT performance of the DG unit under an unbalanced grid fault is investigated using three different reference current generation strategies to determine the best strategy to provide a stable and efficient LVRT performance under strong and weak grid conditions. The time-domain simulation and experimental results are also presented to validate the effectiveness of the proposed methods.